Surgery by the application of radio frequency currents to living tissue to desiccate, cut, or vaporise the tissue using one or more electrodes coupled to a radio frequency generator raises particular problems in terms of obtaining efficient transfer of power from the generator to the tissue and producing a required surgical effect in a controllable manner. The electrical characteristics of an electrode assembly when in use can vary widely depending on the mode of use, the conductivity of the tissue and surrounding substances, and the nature of the assembly itself.
These problems are particularly evident in the case of electrosurgery performed with an electrode or electrodes immersed in liquid at the operation site (often referred to as "underwater" electrosurgery).
Underwater surgery is commonly performed using endoscopic techniques in which (1) the endoscope itself may provide a conduit for passage of an electrode, commonly referred to as a working channel, or (2) the endoscope may be specifically adapted to include means for mounting an electrode, such as are provided on a resectoscope, or (3) the electrode may be introduced to the body cavity via separate access means at an angle with respect to the endoscope; a technique commonly referred to as triangulation. These variations in technique can be subdivided by surgical speciality, where one or other of the techniques has particular advantages given the access route to the specific body cavity. Endoscopes with integral working channels or those characterised as resectoscopes, are generally employed when access to the body cavity may be through a natural body opening; such as the cervical canal to gain access to the endometrial cavity of the uterus or the urethra to gain access to the prostate gland and bladder. Endoscopes specifically designed for use in the endometrial cavity are referred to as hysteroscopes. Those for the urinary tract include cystoscopes, urethroscopes and resectoscopes used during transurethral resection or vaporisation of the prostate gland (TURP and EVAP, respectively). When there is no natural body opening through which the endoscope may be passed, the technique of triangulation is commonly employed. A common site where triangulation is used is during underwater endoscopic surgery on joint cavities such as the knee and shoulder. The endoscope used in these procedures is commonly referred to as an arthroscope.
In our co-pending British Patent Applications Nos. 9512888.0 and 9512889.8 (the disclosure of which is incorporated herein by reference) we disclose an electrosurgical generator and an electrode assembly designed for operation with electrodes immersed in a conductive liquid such as saline solution. The electrode assembly has two electrodes; a first, active electrode at the extreme distal end of the assembly for contacting the tissue to be treated, and a second, return electrode spaced proximally from the active electrode and separated from the latter by an insulation barrier. When the electrodes are immersed in the saline solution, the solution provides a conductive path between the tissue next to the active electrode and the proximal return electrode which remains spaced from the tissue. The electrode assembly is fed by a generator which includes a rapid-acting power reduction circuit operating to prevent significant vapour formation at the active electrode during electrosurgical desiccation. The output power supplied to the electrode assembly by the generator is rapidly reduced when the peak output voltage reaches a preset threshold with the object of avoiding a rapid runaway increase in power delivery and arcing when vaporisation commences, which would lead to uncontrollable tissue disruption in place of the required desiccation. This effect is especially problematical when the generator has a significant output impedance. Different electrode assemblies can be used to perform different electrosurgical functions.
It is an object of the present invention to provide an electrosurgical instrument which produces a controllable surgical effect and which is versatile in use.